The objectives of this project are to (1) determine if genes present on plasmid DNA of Borrelia burgdorferi, the causative agent of Lyme disease, control infectivity, (2) characterize these plasmids, (3) clone the infectivity genes and express those components that promote Borrelia infection in mammals, and (4) determine immunogenic properties of components responsible for infection, and (5) use recombinant DNA techniques to express B. burgdorferi-specific antigens to improve the serodiagnosis of Lyme disease. The reduction in the number of detectable plasmids with the loss of infectivity suggests that gene(s), encoding for components related to infectivity, may be present on one or more of these extrachromosomal elements. A 8.4 kilobase (kb) pair circular plasmid was first identified as a strong candidate for regulating infection. Numerous fresh strains of B. burgdorferi were examined for their infectivity and presence of the 8.4 kb plasmid, two strains that were infectious lacked this plasmid or sequences similar to it elsewhere in their genomes. Thirteen strains of B. burgdorferi were cloned by limiting dilution from the low passage Sh-2-82 and examined for their infectivity, plasmid profile, and reactivity with anti-OspB monoclonal antibodies. One clone was noninfectious and had a unique and deficient plasmid profile compared to the other 12 clones. Investigations concerning in vivo antigenic changes in white-footed mice, the natural reservoir for B. burgdorferi, and white mice are continuing and have demonstrated that antigenic variation during infection is significant and may play an important role in the spirochete's ability to maintain persistent infections in spite of a pronounced humoral immune response by the host.